Heavy or settlement-sensitive facilities that are located in areas containing soft or weak soils are often supported on deep foundations, consisting of driven piles or drilled concrete columns. The deep foundations are designed to transfer the structure loads through the soft soils to more competent soil strata.
In recent years, aggregate columns have been increasingly used to support structures located in areas containing soft soils. The columns are designed to reinforce and strengthen the soft layer and minimize resulting settlements. The columns are constructed using a variety of methods including the drilling and tamping method described in U.S. Pat. Nos. 5,249,892 and 6,354,766; the driven mandrel method described in U.S. Pat. No. 6,425,713; the tamper head driven mandrel method described in U.S. Pat. No. 7,226,246; and the driven tapered mandrel method described in U.S. Pat. No. 7,326,004; the disclosures of which are incorporated by reference in their entirety.
The short aggregate column method (U.S. Pat. Nos. 5,249,892 and 6,354,766), which includes drilling or excavating a cavity, is an effective foundation solution when installed in cohesive soils where the sidewall stability of the hole is easily maintained. The method generally consists of: a) drilling a generally cylindrical cavity or hole in the foundation soil (typically around 30 inches); b) compacting the soil at the bottom of the cavity; c) installing a relatively thin lift of aggregate into the cavity (typically around 12-18 inches); d) tamping the aggregate lift with a specially designed beveled tamper head; and e) repeating the process to form an aggregate column generally extending to the ground surface. Fundamental to the process is the application of sufficient energy to the beveled tamper head such that the process builds up lateral stresses within the matrix soil up along the sides of the cavity during the sequential tamping. This lateral stress build up is important because it decreases the compressibility of the matrix soils and allows applied loads to be efficiently transferred to the matrix soils during column loading.
The tamper head driven mandrel method (U.S. Pat. No. 7,226,246) is a displacement form of the short aggregate column method. This method generally consists of driving a hollow pipe (mandrel) into the ground without the need for drilling. The pipe is fitted with a tamper head at the bottom which has a greater diameter than the pipe and which has a flat bottom and beveled sides. The mandrel is driven to the design bottom of column elevation, filled with aggregate and then lifted, allowing the aggregate to flow out of the pipe and into the cavity created by withdrawing the mandrel. The tamper head is then driven back down into the aggregate to compact the aggregate. The flat bottom shape of the tamper head compacts the aggregate; the beveled sides force the aggregate into the sidewalls of the hole thereby increasing the lateral stresses in the surrounding ground.
The driven tapered mandrel method (U.S. Pat. No. 7,326,004) is another means of creating an aggregate column with a displacement mandrel. In this case, the shape of the mandrel is a truncated cone, larger at the top than at the bottom, with a taper angle of about 1 to about 5 degrees from vertical. The mandrel is driven into the ground, causing the matrix soil to displace downwardly and laterally during driving. After reaching the design bottom of the column elevation, the mandrel is withdrawn, leaving a cone shaped cavity in the ground. The conical shape of the mandrel allows for temporarily stabilizing of the sidewalls of the hole such that aggregate may be introduced into the cavity from the ground surface. After placing a lift of aggregate, the mandrel is re-driven downward into the aggregate to compact the aggregate and force it sideways into the sidewalls of the hole. Sometimes, a larger mandrel is used to compact the aggregate near the top of the column.
U.S. Pat. No. 7,604,437 is related to a mandrel for making aggregate support columns wherein flow restrictors are provided to prevent upward movement of aggregate through the mandrel during driving of the mandrel. The mandrel contemplated in this art relates to formation of an aggregate support column such as described in U.S. Pat. Nos. 6,425,713 and 7,226,246 discussed above.
U.S. Pat. Nos. 4,992,002 and 6,773,208 relate to methods for casting a partially reinforced concrete pier in the ground. One method involves the use of an elongate mandrel with a cupped foot having a larger cross-sectional area than the mandrel, wherein flowable grout that is placed in the mandrel flows through openings located near the bottom of the mandrel into the space between the mandrel and the foot. The other method involves the installation of an elongate hollow tubular casing that is then filled with fluid concrete that is allowed to set while the casing remains in the ground. Each of these references is merely to concrete hardened inclusions and does not allow for the additional stability and strength provided by a pier that has an expanded base.
In the area of soil improvement, it is often desirable to install a stiff inclusion into the ground to transfer loads through a soft or weak soil layer. Although these soil layers may also be treated by non-cementitious aggregate columns, non-cementitious columns are typically confining-stress dependent (i.e., they rely on the strength of the sidewall soils to prevent bulging). Occasionally, it is desirable to utilize cementitious inclusions to bypass weak soils and transfer loads to underlying strong soils. The object of the present invention is to efficiently form a strong and stiff expanded base (either cementitious or non-cementitious) at the bottom of the column and to provide an efficient means for the introduction of grout, concrete, post-grouted aggregate, or other cementitious material through the upper portions of the column to form a cementitious inclusion.